Optical data storage disks have gained widespread acceptance for the storage, distribution and retrieval of large volumes of information. Optical data storage disks include, for example, audio CD (compact disc), CD-R (CD-recordable), CD-RW (CD-rewritable) CD-ROM (CD-read only memory), DVD (digital versatile disk), DVD-R, DVD-RW, DVD-ROM, DVD-RAM (DVD-random access memory), HD-DVD (high definition-DVD), Blu-Ray, and various other types of writable or rewriteable media, such as magneto-optical (MO) disks, phase change optical disks, and others.
Optical data storage disks are typically produced by first making a data storage disk master that has a surface pattern that represents encoded data and/or precision tracking features on the master surface. The surface pattern on the master disk, for instance, may be a collection of precisely dimensioned grooves or other features that define master pits and master lands, e.g., typically arranged in either a spiral or concentric manner. The master is typically not suitable as a mass replication surface, as the master features are typically defined within an etched photoresist layer formed over a master substrate.
After creating a suitable master, that master can be used to make a stamper, which is less fragile than the master. The stamper is typically formed of electroplated metal or a hard plastic material, and has a surface pattern that is the inverse of the surface pattern encoded on the master. An injection mold can use the stamper to fabricate large quantities of thermoplastic replica disk substrates. Also, photo-polymerization replication processes, such as rolling bead processes, have been used to fabricate replica disk substrates using stampers. In any case, each replica disk substrate may contain the data and tracking precision that was originally encoded on the master surface and preserved in the stamper. The replica disk substrates can be coated with other materials, such as dyes, phase change materials, and/or reflectors to form the optical disks.
Some types of optical data storage disks contain multiple substrates that are bonded together. For example, DVD, DVD-ROM, DVD-R, DVD-RW, and HD-DVD media constructions conventionally include two substrates, both having a thickness of approximately 0.6 millimeters. Typically, one of the substrates bears the data and/or tracking information, while the other non-information bearing substrate provides the functions of backside protection, reference clamping area, a labeling area, and improved mechanical stability. In these types of bonded media constructions, the information-bearing surface of one of the substrates is sandwiched between two substrates. As such, the information bearing surface is interrogated by a drive system using a beam of laser light that is brought to focus through the information bearing substrate. This, in turn, places strict optical requirements for thickness, thickness uniformity, index of refraction, and birefringence for the information-bearing substrate.
The non-information bearing “dummy” substrate, in conventional DVD constructions, provides backside protection insofar as the information bearing surface is encased between the two bonded substrates. In addition, the non-information bearing substrate also functions to improve robustness of the medium by improving both the dynamic and static mechanical stability. A bonded non-information bearing substrate improves static mechanical stability by minimizing bending stress to the information-bearing surface. Likewise, a properly bonded non-information bearing substrate improves dynamic mechanical runout for the rotating media.
With DVD media constructions (such as DVD, DVD-ROM, DVD-R, DVD-RW, and HD-DVD), the data layer is located approximately 0.6 millimeters in from the surface of the disk closest to the laser. During reading and recording, the laser beam passes through the portion of the disk located between the laser and the data layer. This portion of the disk, the optically functional portion, must adhere to specified requirements in regard to optical thickness, optical thickness uniformity, allowable thickness range, allowable birefringence, and range of optical index of refraction for the material.